docs: def import_app_credentials(filename=CREDENTIALS_FILENAME):
      """Import app credentials from configuration file.
  
      Parameters
          filename (str)
              Name of configuration file.
  
      Returns
          credentials (dict)
              All your app credentials and information
              imported from the configuration file.
      """
      with open(filename, 'r') as config_file:
          config = safe_load(config_file)
  
      client_id = config['client_id']
      client_secret = config['client_secret']
      redirect_url = config['redirect_url']
  
      config_values = [client_id, client_secret, redirect_url]
  
      for value in config_values:
          if value in DEFAULT_CONFIG_VALUES:
              exit('Missing credentials in {}'.format(filename))
  
      credentials = {
          'client_id': client_id,
          'client_secret': client_secret,
          'redirect_url': redirect_url,
          'scopes': set(config['scopes']),
      }
  
      return credentials